Solar-powered sensors to harvest almost double the energy

By Laura Onita

Published Wednesday, June 24, 2015

A new power converter chip that can harvest more than 80 per cent of the energy trickling into it has been designed by MIT researchers.

Previous ultra-power converters had efficiencies of only 40 or 50 per cent, and where its predecessors could use a solar cell to charge a battery or directly power a device, the new chip can do both, the researchers said. It can power the device directly from the battery.

The chip could forge the way for extremely low-power sensors that can run for months without battery changes or that can extract energy from the environment to recharge, which is necessary in the Internet of Things age. Vehicles, appliances and equipment would have their own embedded sensors to feed back information directly to network services, streamlining the maintenance process and coordination of tasks.

The chip's operations share a single inductor, its main electrical component, which saves space on a circuit board, but increases the circuit complexity even further while keeping the power consumption low.

“We still want to have battery-charging capability, and we still want to provide a regulated output voltage,” said Dina Reda El-Damak, an MIT graduate student in electrical engineering and computer science and first author on the paper. “We need to regulate the input to extract the maximum power, and we really want to do all these tasks with inductor sharing and see which operational mode is the best. And we want to do it without compromising the performance, at very limited input power levels — 10 nanowatts to 1 microwatt — for the Internet of Things.”

The circuit’s role is to regulate the voltages between the solar cell, the battery and the device the cell is powering. If the battery operates at wrong voltages it loses the ability to hold charge. To address this and control the current flow across the chip, the MIT researchers used an inductor to prevent any changes in the current by creating a magnetic field.

Throwing switches in the inductor’s path causes it to alternately charge and discharge, so that the current flowing through it continuously ramps up and then drops back down to zero. Keeping a lid on the current improves the circuit’s efficiency, since the rate at which it dissipates energy as heat is proportional to the square of the current.

Once the current drops to zero, however, the switches in the inductor’s path needs to be thrown immediately; otherwise, current could begin to flow the circuit in the wrong direction, which would drastically diminish its efficiency. To control the timing of the switches’ throws, the engineers used a capacitor to store electrical charge. The higher the current, the more rapidly the capacitor fills. When it’s full, the circuit stops charging the inductor.

Equipping their chip with a bank of capacitors of different sizes, as the current drops it charges a subset of those capacitors, whose selection is determined by the solar cell’s voltage. Once again, when the capacitor fills, the switches in the inductor’s path are flipped.